1. Field of the Invention
The present invention relates to video telephone systems. In particular, the invention relates to adaptive control of a CCD camera to simplify and speed computations needed in a compression engine such as discrete cosine transforms of video conference standard H.263 or an MPEG compression engine.
2. Description of Related Art
CCD cameras are known. A CCD camera includes a CCD imager IC (a large CCD integrated circuit) and control circuits typically mounted on a printed wiring assembly. The CCD imager IC includes a two dimensional array of photosites. In a CCD imager IC, the photosites are exposed to an imaging fight for a short integration period. At the end of the integration time, the charge accumulated in the photo sites are typically parallel transferred rapidly to a plurality of vertical shift registers (protected from further exposure to light) and then serially shifted down to a horizontal shift register. The end element of each vertical shift register forms a plurality of parallel signals. This plurality of parallel signals are rapidly parallel transferred into the horizontal shift register. From there, the horizontal shift register is clocked serially to transfer the charge to an output circuit for the CCD imager IC. When all elements of the horizontal shift register are transferred out of the CCD imager IC, the plurality of vertical shift registers are again clocked once to make a new vertical shift register element available from each vertical shift register for parallel transfer into the horizontal shift register. By this process, the CCD imager IC produces a serial signal that corresponds to a raster scan such as for an NTSC compatible signal.
When such a CCD camera is used with an H.263 or MPEG compression engine, compressed video results. Since CCD imager ICs are typically designed for compatibility with NTSC (or PAL for Europe) signal generation, the H.263 or MPEG compression engine is designed to use the input of the NTSC signal. The CCD imager IC has a variety of control parameters that must be specified and are usually controlled by a signal from outside of the CCD imager IC. For example, the integration time, and the clock rates of the vertical and horizontal shift registers. A number of anti-blooming and charge reset features are usually provided, and these too are controlled by signals from outside of the CCD imager IC. In color cameras, even more control is effected from outside the imager IC. Sometimes the imager IC architecture provides even more flexibility, such as the selective control to average a 2 by 2 array of pixels to reduce resolution and reduce output bandpass requirements. Known CCD cameras uses these control signals to selectively switch between, for example, an NTSC signal and a PAL signal, or between black and white and color.